Along with the greatly increased use of computers for offices and for manufacturing facilities, there developed a need for a cable which may be used to connect peripheral equipment to mainframe computers and to connect two or more computers into a common network. Of course, given the ever-increasing demands for data transmission, the sought-after cable desirably should not only provide substantially error-free transmission at relatively high bit rates or frequencies but also satisfy numerous other elevated operational performance criteria. Specifically, the particular cable design of the present invention consistently performs at operational levels which exceed the transmission requirements for cables qualifying as Category 5 cables under TIA/EIA-568A. The particular operational performance aspects that the cable design of this invention can reliably and consistently enhance over existing cables, include the degree to which the insertion loss and characteristic impedance value of one conductor-pair is matched to the insertion loss and characteristic impedance values of the other conductor-pairs within the same cable.
Not surprisingly, of importance to the design of metallic-conductor cables for use in local area networks are the speed and the distances over which data signals must be transmitted. In the past, this need had been one for interconnections operating at data speeds up to 20 kilobits per second and over a distance not exceeding about 150 feet. This need was satisfied with single-jacket cables which may comprise a plurality of insulated conductors that were connected directly between a computer, for example, and receiving means such as peripheral equipment. Currently, equipment, generally identified throughout the industry as Category 3 products, is commercially available that can effectively transmit up to 16 MHz data signals and a series of products designated as Category 5 provide the capability of effectively transmitting up to 100 MHz data signals.
The objectives being demanded by cable customers, including local area network (LAN) vendors and distribution system vendors, are becoming increasingly stringent. This is true for both the breadth of the types of features demanded as well as the technical wherewithal necessary to accomplish the new requests from customers. In this regard, further advances in the operational performance of LAN cables are becoming increasingly difficult.
The unshielded twisted pair has long been used for telephone transmission in the balanced (differential) mode. Used in this manner, the unshielded twisted pair has excellent immunity from interference whether from the outside (EMI) or from signals on other pairs (crosstalk). Another point of concern with the use of such cables is that each cable be designed so as not to emit electromagnetic radiation from the cable into the surrounding environment. Over the past several years, in fact, some LAN designers, have come to realize the latent transmission capability of unshielded twisted pair wire. Especially noteworthy is the twisted pair's capability to transmit rugged quantized digital signals as compared to corruptible analog signals.
In an attempt to enhance the operational performance of twisted pair cables, manufacturers have employed a variety of different twist schemes. As used herein, twist scheme is synonymous with what the industry sometimes calls twinning or pairing. In general, twist scheme refers to the exact length and type/lay of twist selected for each conductor pair. More specifically, in one such twist scheme particularly described in commonly-assigned U.S. Pat. No. 4,873,393 issued in the names of Friesen and Nutt and which is hereby expressly incorporated by reference, it is stated that the twist length for each insulated conductor pair should not exceed the product of about forty and the outer diameter of the insulation of one of the conductors of the pair. While this is just one example of an existing approach for defining a twist scheme which results in an enhanced cable design, many others exist.
As a more recent piece of prior art, the reader's attention is drawn to a unique twist scheme set forth in commonly-assigned patent application filed in the names of Friesen, Hawkins and Zerbs on Jan. 31, 1997 and which is expressly incorporated by reference herein. This document describes a particular series of conductor-pair twist lengths that when used together in a single cable provide operational performance values that significantly surpass the requirements of TIA/EIA-568A.
However, in addition to controlled pair twist schemes, another treatment for crosstalk is to add shielding over each twisted pair to confine its electric and magnetic fields. However, as the electric and magnetic fields are confined, resistance, capacitance and inductance all change, each in such a way as to increase transmission loss. For instance, it is not unusual to find designs of shielded pairs whose attenuation is three times that of similar unshielded pairs. Even in light of these positions regarding shielded cables, it should be understood by the reader that a cable can benefit from the teachings of this document whether the sheath system of the cable includes a shielding element of some type or not.
Notwithstanding the aforementioned problems and solutions, there still appears to be a need for a cable that satisfies the criteria discussed above and also addresses the need for communication cables, particularly LAN cables, to provide more consistent insertion loss and characteristic impedance values between the various conductor-pairs within a single cable.